This invention relates to a microscopic length scale for calibrating the magnification of electron beam instruments such as SEMs, electron and ion microprobes and electron beam writing devices.
The magnification of an SEM, for example, is calibrated by placing an artifact of known dimensions in the instrument and measuring the magnified dimension of the artifact. If the artifact is known to be 10 .mu.m, for instance, and its video image measures 9.9 mm, the magnification is 9900/10 = 990.times.. In this example, the nominal magnification of the SEM would probably read 1000.times. and the information would be used to construct a calibration curve, or the SEM's magnification controls would be trimmed to produce a video image precisely 1000 .times. 10 .mu.m = 10 mm long.
Prior artifacts include gold-coated latex spheres and gold-coated carbon replicas of cross-ruled spectrographic gratings. The diameters of the latex spheres are difficult to measure and tend to change when the spheres are heated by the SEM electron beam. Thus the sphere diameters during a calibration are highly uncertain. The dimensions of the spectrographic gratings tend to change during each successive replication and it is difficult to clean the gratings of the contamination due to the electron beam striking any vacuum oil film thereon.
It has been proposed to use alternating layers of two semiconductor materials such as gallium arsenide and gallium arsenide phosphide as a resolution indicator. However, these materials are not stable when placed in an SEM and hence cannot provide a useful length scale.